A familiar technique for increasing the sensitivity to linear motion of a grating-type translation sensor is to observe the moire pattern interference between one fixed grating and another moving one having a slightly different effective pitch. Accurate sensing of the light level transmitted by the grating pair at some reference point makes it possible to interpolate the magnitude of the motions, which are only a fraction of the grating pitch interval. However, accurate sensing of small changes in light is limited by amplifier noise and drift and instability of the light source.
The present invention is intended to overcome or at least ameliorate these problems, as will become apparent as the description proceeds.